Trillions of cicadas are coming to the U.S. Here’s why that’s a good thing. Don’t be afraid or annoyed of the coming periodical cicadas. It’s a once-in-a-17-year chance to enjoy a wondrous natural phenomenon. Adult Brood X cicadas, which come out only once every 17 years, on a wooden fence in Maryland, in 2004. ByDouglas Main Email In the late spring of 1634, pilgrims in Massachusetts witnessed an incredible sight: Millions and millions of winged, red-eyed insects sprung from the earth. Not knowing better, the Puritan immigrants likened them to pestilential swarms from the Old Testament and called them “locusts.”